1. Field of the Invention
The present invention relates generally to the detection of high energy radiation. The present invention relates more particularly to the film bulk acoustic wave resonator-based devices, and their use in the detection of high energy radiation.
2. Technical Background
Ionizing radiation sensing has a wide range of applications. High dose radiation measurements are important for understanding the degradation of silicon-based electronics in large high energy physics experiments [1]. Low dose sensing is essential in nuclear materials detection and security, where the sources may be partially shielded [2]. Measuring the flux and direction of high energy radiation with space-based detectors is important in the field of astronomy [3]. Determining the precise location and amount of radiation incident on a tumor or other tissues is critical in radiation therapies used in cancer treatment [4].
There are a number of different types of sensors used in the detection of radiation. For very high precision, gamma ray spectrometry can be used [5] but these devices are often extremely expensive and large in size. Lithium fluoride or calcium fluoride thermoluminescent dosimeters (TLDs) are often used in biomedical applications [6]. In order to determine the received dose after exposure, the TLD is heated and the amount of light emitted is measured using a spectrometer. These dosimeters are relatively inexpensive but require significant post-processing to determine the absorbed dose.
In biomedical applications, miniature devices are especially important because they are less invasive and can be placed closer to the target location and thus give more accurate results. Solid-state MOSFET dosimeters (RADFETs) are active devices that have been developed over the past decade and have high sensitivities, but when used in wireless communications require a large inductor which limits the ability to miniaturize the device [7]. Existing MicroElectroMechanical Systems (MEMS) research employs electret-based variable capacitors for measuring radiation, where the electric field generated by the electret collects charge from ionizing radiation in an air gap, reducing the surface charge density which varies the force on the movable plate [8-9]. This capacitor also needs to operate in conjunction with an inductor and by monitoring the resonant frequency of the circuit the signal dosage is determined Therefore, the sensor size due to large inductor, with its low quality factor, limits the ability of integration, and sensitivity may also be affected by ions released into the air gap during fabrication (wall effect).